Modulated high frequency transmitter system



I 7. E. KRAMAR ETAL 2,212,568

HODULATED HIGH FREQUENCY TRANSMITTER SYSTEM Original Eiled Aug. 21, 1935 2 Sheets-Sheet I In ventors: fins/Kramer [rim/famed? 2 Sheets-Sheet 2 v Aug. 27, 1940. E. KRAMAR ET AL HODULATED HIGH FREQUENCY TRANSMITTER SYSTEM Origihal Filed Aug. 21, 1935 PatentedbAu g. 27, 1940 MODULATED HIGH FREQUENCY TRAN S- MITTER SYSTEM' Ernst Kramar and Erich Heinecke, Berlin, Germany, assign'ors to C. Lorenz Aktiengesellschaft, Berlin-Tempelhof, Germany, a. com

Original application August 21, 1935, Serial No. 37,122. Divided and this application July 9, 1938. Serial No. 218,466. In Germany February 5, 1935 4 Claims. (01. 179-1715) In various cases, such as with sound modulated transmitters for feeding radio beacons, the problem arises of keeping the degree of modulation constant. Let first the problem be briefly explained which exists for instance with radio beacons. It has been proposed with a radio beacon for ultra short waves to feed a dipole continuously from the transmitter andto key two reflector dipoles alternately in the dot-and-dash rhythm. This beacon arrangement if erected at a suitable place may be used not only for lateral direction finding but also for landing according to the slip-way method. However, the application of the slip-way method which as is well known works with anintensity indication, the descent being along a curve of constant field intensity, requires that on the receiving side the loudness be not subject to variations. Since the loudness for a constant amplitude of the modulation frequency depends on the modulation degree the problem here is to keep constant the amplitude of the frequency which is to be modulated. In order, moreover, to obtain a large range of transmission, while ensuring that the transmitter be well utilized, the transmitter is modulated very intensely, namely 'to a degree beyond 90%.

It is relatively simple to maintain the constancy of a modulation frequency employed for characterizing the beacon, such as a frequency of say 1000 c. p. s. The amplitude of the carrier frequency however is less easy to maintain constant. If now for instance the amplitude of the carrier frequency decreases while the modulation frequency continues to be of a constant amplitude, then an overmodulation occurs because as stated a very intense modulation is provided'for. Although in this way the transmitter will not in any case be fully overmodulated yet at least harmonies or over-tones will occur, which interfere very seriously with the navigation. As is well known, in addition to the main beacon so-called pre-signals are employed with radio beacons, such pre-signals being based for instance on the same wavelength while working with different characteristic tones. These prez-signals serve for example to characterize the commencement of the landing or serve to indicate the boundary of the landing ground. If now harmonic waves arrive due to overmodulation of the beacon transtioned phenomena of solved by controlling either the bias of a stage connected inadvance of the modulation stage or the bias of the modulation stage itself.

Two embodiments of the invention are shown by way of example in the accompanying draw- '5 ings,

Fig. 1 being a diagrammatic representation of one of these embodiments,

Fig. 2 a diagrammatic representation of the other embodiment, v

Figs. 2a and 2b represent modifications of Fig. 2, only so much of the circuit of Fig. 2 being represented as is necessary to illustrate the modifications therein.

In Fig. 1 the first three stages I, II, III of a multistage transmitter are shown.

The first stage I is arranged to operate with I quartz control; in the second stage II a control for effecting the constancy of the modulation degree is active, while in the grid circuit of the third stage III the modulation is effected. The modulation frequency is impressed over the transformerT upon the push-pull tubes Rm by grid voltage modulation. In the anode circuit an inductance L1 is connected which islocated in the grid circuit of the next stage. Further, through" the inductance Ina rectifier G, which in the case represented is a tube the grid of which is connected to the anode, is coupled. The rectified voltage is used for controlling the grid bias of the second stage over a condenser Cg and a choke Dg. The control is efiected with the aid of the resistance V- and W. Included in series with V is a choke D11. At GV a constant negative bias is given. The other parts represented in the 5 drawings are well known. They have per se nothing to do with'the invention.

The mode of action of the arrangement is as follows: The modulation frequency arriving over T is practically constant. The carrier frequency already modulated is rectified in the output cir-- cult of the third stage by the rectifier G. The resultant continuous voltage is a means for measuring the magnitude of the carrier, for as is well known the mean value of a modulated oscillation does not vary if the transmitter is otherwise constant. The voltage relations are now so selected that the grid bias of the stage II shall always be So controlled that the carrier amplified in stage 11 shall always/be of the same magnitude. The only requirement is that the control is effected in a stage in which the magnitude of the carrier alone can be influenced, as otherwise the menover-modulation cannot be avoided. j

I may of course be used. Furthermore, the control may of course be effected in another manner customary in the high frequency art.

The further invention, to which Fig. 2 relates, proposes to rectify part of the non-modulated carrier frequency and to use the resultant continuous voltage for controlling the bias of one of the prestages of the transmitter. In order to effect this method, preferably a tube rectifier is employed whose control grid is attached to the high frequency potential of the anode whilst the control grid is at the same time given a continuous current bias.

This will be understood from the following description which also states the advantages due to rectifying the non-modulated carrier frequency.

In Fig. 2 four stages of a multistage transmitter are shown. In stage I a quartz control is employed. In the stage II a control for efiecting the constancy of the modulation degree is active, whilst in the grid circuit of the stage IV the modulation is accomplished. .In the output of the stage III the continuous control voltage is derived by the rectifier G. The modulation frequency is impressed over 'the transformer T upon the push-pull tubes RIV by grid voltage modulation. The rectifier G is coupled through a condenser C. The continuous voltage which the rectifier G produces at the resistance W across the choke coil D1 is laid, for instance through a resistance V and a choke D3, to the grid of the tube of stage II. The grid of the rectifier tube is caused by the condenser K to have the same high frequency potential as has the anode, and hence is short-circuited with respect to the high'frequency. Over the choke Dz a constant continuous voltage is given to the grid of the rectifier. nected directly with the anode then the grid bias of the stage II, i. e. the grid bias having to be regulated in order to ensure a reliable operation of the transmitter, and the setting or adjusting of the rectifier working point, would not be independent of each-other. This is avoided by the provision that the grid of the rectifier is caused by condenser K to have the high frequency potential of the anode, while over the choke D2 continuous voltage is available for enabling the rectifier working point to be set at will, so that a regulation of the stage II by the rectified carrier occurs only at a definite value, that isto'say, it is this value which is kept constant. Over the resistance W a constant continuous voltage is led to the grid of the stage II as shown.

The bias on the grid of the rectifier may be controlled continuously, such as by the modulation tone or independency upon the network voltage. In the first case one has to rectify the modulation tone and to employ the resultant continuous voltage for controlling the grid.

In the foregoing description referring to Fig. 1, in order to maintain the modulation degree constant, the assumption has been made that the modulation ,tone is in itself of constant amplitude, this being in many cases obtainable by simple means. If such is not the case however, then for rendering the loudness in the receiver constant the modulation tone may as stated be rectified and thus the bias of the rectifier be controlled. The field intensity of the transmitter, it is true, will then not be constant, but at least the modulation degree will be so, so that varia- If in the usual manner the grid were con-.

tions in the loudness will be prevented with the receiver adjusted for a definite sensitivity.

Fig. 2a schematically illustrates a modification of Fig. 2 suitable for embodying this feature of the invention. In this figure G2 represents rectifying means connected to rectify a portion of the tone frequency supplied across the primary of transformer T. It will be noted that the output of such rectifying means is applied to the grid return lead of rectifier tube G, thus replacing the grid bias source schematically represented by -G.V in Fig. 2. With this form of the invention, if the amplitude of the tone supplied to transformer T increases a corresponding'increase of negative bias on rectifier G will result which in turn, by decreasing the current through resistor W,- will reduce the negative bias of stage II, thus giving a compensatory increase or carrier amplitude so that the depth of modulation will remain constant even though the field strength and percentage of modulation vary. v

Fig. 212 represents another embodiment of a plicants invention wherein the bias of rectifier tube G is controlled by the same method but with respect to variations in the network voltage rather than with respect to variations in the amplitude of the tone. In this Fig. 2b the rectifying means G2 is connected in similar manner to the rectifying means G2 previously described, but the input thereof is fed by the network voltage rather than by the tone frequency across transformer T. This arrangement thereforserves to compensate for variations of network voltage rather than for variations of amplitude of the tone.

This application is a division of application Serial No. 37,122, filed August 21, 1935, and issued as U. S. Patent No. 2,151,921.

What is claimed is:

1. A modulating system, comprising carrier current producing means, a carrier current amplifier tube having a control grid, modulating means connected to the output of said amplifier tube, a rectifier tube having a control grid and an anode arranged to rectify a portion of the unmodulated output current of said amplifier tube, means for causing resulting rectified current to apply a corrective bias to the grid of said amplifier tube to hold constant the amplitude of said carrier current, means to cause said control grid of said rectifier tube to have the same high frequency potential as said anode whereby the adjustment of the rectifier to a desired working point can be effected independently of the adjustment of the normal grid-bias of said amplifier tube, and means for giving the said control grid a continuous current bias;

2. A system according to claim 1, wherein means is provided for independently adjusting said continuous current bias of said rectifier tube.

3. A system according to claim 1, wherein means is provided for adjusting said continuous current bias of said rectifier tube in accordance with the modulation of the transmitter so that the amplitude of the modulated current will be maintained constant.

4. A system according to claim 1, wherein means is provided for adjusting said continuous current bias of said rectifier tube in accordance with the feeding voltage of the system so that the amplitude of the modulated current will be maintained constant.

ERNST KRAMAR. ERICH HEINECKE. 

